The invention relates to telecommunications systems for transmitting digital data. It relates more particularly to techniques for receiving high bit rate digital signals transmitted over long-haul optical fiber connections, for example.
A transmission system typically includes one or more transmitters connected to a receiver via a connection which, in the case of an optical signal, may consist of a single fiber and/or a more complex connecting medium including optical amplifiers and switches constructed from couplers, waveguides and optical gates, for example.
FIG. 1 is a diagram of one example of a transmission system. An optical transmitter TX is coupled to one end of an optical connection L, here via an optical amplifier 4, to inject an optical signal OSe into it. The other end of the connection, coupled to an optical receiver RX, delivers to it an optical signal OSr resulting from the transmission of the send signal OSe.
Information to be transmitted is conventionally supplied to the transmitter TX by a station (not shown) and is generally in the form of digital data De organized into successive bits. The data is applied to a parallel interface of the transmitter. Starting from that interface, the transmitter includes a processing system comprising in succession a transcoder module 1, a parallel-to-serial converter 2 and an electrical-to-optical converter 3. These components code the digital data De and convert it into a serial binary signal Se in the form of a modulated electrical signal which is then converted into an optical signal OSe by modulating an optical carrier wave as a function of the electrical signal Se.
In order to use wavelength division multiplexing as well, the system is equipped with a multiplexer (not shown) between the converter 3 and the amplifier 4 for combining a plurality of signals originating from a plurality of transmitters and carried by different wavelengths. In this case, it is also necessary to provide a demultiplexer (not shown) between the receive end of the connection and each receiver.
The receiver RX includes a processing system comprising in succession an optical-to-electronic converter 5 receiving the optical signal OSr and supplying a modulated electrical signal Sr, an analog-to-digital converter 6 forming from the signal Sr a serial binary signal Sb, a serial-to-parallel converter 70, and a decoder 8 for supplying received digital data Dr corresponding to the sent digital data De.
The parallel-to-serial converter 2 of the transmitter creates a modulated electrical signal Se at the timing rate of a send clock Ck in order to write bits in successive time periods each having a duration equal to the clock period. The send clock frequency (also known as the bit frequency) determines the transmission bit rate.
The electrical signal Se is obtained by modulating the amplitude of an electrical parameter (typically the voltage) in accordance with one of the standard modulation formats, generally of the NRZ or RZ type. The signal Se therefore carries successive bits in the form of modulation between low and high amplitude levels respectively representing first and second bit values, here designated “0” and “1”, respectively, in accordance with the usual convention.
Converting the signal Se into an optical signal OSe consists in modulating the carrier wave, generally modulating its amplitude, although its phase or its optical frequency, or a combination of the above physical parameters, can be modulated.
The optical-to-electronic converter 5 of the receiver RX is adapted to the type of modulation selected for sending in order to supply a modulated electrical signal Sr that reproduces the send electrical signal Se. Like the send signal Se, the receive signal Sr carries successive bits in the form of modulation between low and high amplitude levels respectively representing binary values 0and 1, according to the convention.
The device 6 for converting the received signal Sr into a digital serial signal must implement two main functions: recovering the timing of the send clock CK from the received signal Sr, and comparing the amplitude of the receive signal Sr to a decision threshold level, the result of this comparison being taken into account during sampling windows synchronized by the recovered clock timing and phase-locked to the signal. As a result of this comparison, a serial binary signal Sb, i.e. a signal modulated in time with the clock CK, is obtained that is able to assume a first or a second discrete state during each bit period, these two states (which are typically first and second fixed voltage levels) respectively representing low and high amplitude levels of the receive signal Sr, i.e. respectively representing first and second binary values 0and 1, according to the convention.
The binary signal Sb may then be processed by standard digital circuits farther downstream in the receiver using the recovered clock (in particular the serial-to-parallel converter 7 and the decoder 8).
The clock timing recovery and comparison functions are implemented by a circuit usually called the “decision circuit” in which the value assigned to a decision threshold level is a critical parameter for correct recognition of the successive binary values received after transmission. In favorable circumstances (stable and calibrated sending sources, short transmission distances, low bit rate, low optical noise, little degradation by the optical connection), this value may be fixed, and is generally made substantially equal to the mean of the low and high levels of the receive signal Sr.
On the other hand, the device 6 will have to include control means for automatically compensating the fluctuations if the low and high levels are liable to fluctuate strongly, for example according to the source that sent the signal, as a function of the path that it has taken in the network, or following variations in optical noise or distortion of the signal caused by non-linear effects.
To this end, prior to the comparison, the receive signal Sr may be processed by an amplifier associated with an automatic gain control system which adjusts the mean amplitude (or DC component) of the signal to a constant level, for example an integrator filter type analog circuit which receives the signal Sr at its input and supplies at its output a gain control signal that is applied to a variable gain amplifier.
Another solution is to use a circuit similar to the above circuit to maintain the threshold level at an optimum value allowing for the mean amplitude of the receive signal Sr.
The drawback of the above methods is that it is difficult to implement the analog circuit with sufficient adjustment accuracy. Moreover, its bandwidth and time constants must be optimized as a function of the bit rate, which does not facilitate upgrades by increasing the bit rate of an already installed system.
Another approach is to exploit the fact that digital data transmission systems often employ forward error correction (FEC) methods to calculate in real time the error rate affecting received data. Error detector and corrector codes are conventionally used for this, and the transmitter calculates redundant data as a function of the selected code and the information data De to be sent, and that redundant data is then sent with the information data. The redundant data is calculated for successive blocks of information data and then combined with them to form frames that are sent successively.
The data of each frame received by the receiver is processed by an error detection and correction module to calculate error syndromes representing the locations of any errors detected and to correct them. This module supplies other information, such as the number of errors detected, the error rate, etc. The redundant data is calculated in the transcoding module 1 shown in FIG. 1, for example. The error syndromes are processed in the decoder 8, for example.
In this context, the threshold level may be adjusted automatically by seeking an optimum value that minimizes the error rate. One embodiment uses an optimization algorithm and the parameters to be minimized are successive values of the error rate measured for the successive frames received. This method may prove disappointing, however, as the algorithm may converge toward local minima of the error rate that do not correspond to the optimum threshold value.
The invention exploits a general property of binary transmission whereby, statistically speaking, the number of 0 bits and the number of 1 bits sent are substantially equal. This approach is particularly suitable for many transmission systems that employ scrambling of the binary data prior to transmission and corresponding descrambling on reception.
The aim of this scrambling, which is effected in the transcoding module 1 shown in FIG. 1, for example, is that the binary data sent should be such that the modulated electrical and optical signals sent, and therefore received, resulting therefrom have particular properties that facilitate their processing by the receiver.
One of those properties is the fact that in any sequence of data consisting of a minimum number of predefined bits, the number of 0 bits must remain close to the number of 1 bits. This ensures that the signal transmitted has a mean value that is independent of the information transmitted over any time period at least equal to the transmission time corresponding to this minimum number of bits. Another example of a property aimed at facilitating clock timing recovery is the fact that the series of consecutive 0 bits and the series of consecutive 1 bits are of limited duration.
In the light of the above remarks, the invention proposes a new method of determining the decision threshold level that avoids the drawbacks of the solutions discussed above.